Project Summary Heterotopic ossification (HO), the formation of bone in skeletal muscle and associated soft tissues, can result from traumatic injury or disease. The most extreme form of HO is manifested in the rare, autosomal-dominant genetic disorder, fibrodysplasia ossificans progressiva (FOP), in which HO continues progressively throughout life, resulting in devastating effects on health, life expectancy and quality of life. We developed a new genetic mouse model of FOP based on conditional expression of the disease-causing BMP receptor, ACVR1(R206H). Using this model, we identified fibro/adipogenic progenitors (FAPs), a multipotent mesenchymal progenitor of muscle tissue, as the offending cell population that gives rise to the heterotopic skeleton. Our genetic studies indicate that the wild type (WT) ACVR1 receptor functions as a direct or indirect competitive inhibitor of ACVR1(R206H) in heterozygous cells. Based on these findings, the overarching hypothesis that provides the conceptual framework and justification for this exploratory grant posits that WT and mutant ACVR1 compete for limiting osteogenic signaling components and that disease severity is dictated by the stoichiometric balance of these receptors. The primary experimental objective of the current proposal is to determine whether ACVR1 over-expression mitigates the deleterious effects of ACVR1 (R206H) in FOP mice, a result that would provide proof-of-concept for a novel and non-obvious therapeutic approach for FOP. Using a newly developed mouse knockin line that conditionally over-expresses WT human ACVR1 (R26ACVR1), Aim 1 proposes functional studies that utilize quantitative ?CT imaging and histological analyses to determine whether ACVR1 over-expression in FAPs effectively inhibits HO. Cell transplantation studies will determine whether ACVR1 over-expression can function cell-non-autonomously, perhaps by binding key osteogenic ligands that drive ACVR1(R206H) signaling. As a sensitive test of the neutralizing effects of ACVR1 receptor over-expression on ACVR1(R206H) function, Aim 1 will also determine whether global embryonic over-expression of ACVR1 can rescue the neonatal-lethal phenotype of mice that broadly express Acvr1R206H. Since severe muscle loss can be a significant contributing factor to patient morbidity, Aim 1 will determine whether ACVR1 over-expression restores muscle regenerative capacity. Aim 2 will use RNA sequencing to define the FAP mRNA transcriptome at early times after injury to identify new direct or indirect transcriptional targets that are associated with entry of multipotent FAPs into the endochondral pathway, and to define the extent to which over-expression of ACVR1 ?normalizes? the FAP transcriptome. Finally, transcriptome analysis of both normal and mutant FAPs derived from pathogenic FOP muscle will address the relative extent to which environmental and intrinsic genetic factors dictate FAP cell fates and transcriptional outcomes. The proposed research will contribute significantly to an understanding of molecular mechanisms of pathological FAP reprogramming and may lead to the development of novel therapeutic strategies for FOP based on ACVR1 over-expression.